Tactical Air Navigation (TACAN/DME) is used worldwide to provide aircraft with range and azimuth information relative to fixed ground stations. Predominantly used in military applications, TACAN also has utility in commercial applications. Equipment implementing only a ranging function, and not an azimuth determination function, is called distance measuring equipment (DME). As such, DME is a variant or subset of TACAN and refers to the ranging function. DME is commonly used by commercial aircraft.
TACAN/DME up-link and down-link signals consist of pairs of gaussian-shaped 3.5 microsecond wide pulses. These signals are pulses of radio frequency (RF) energy transmitted in pairs on 252 possible channels in the 960-1215 MHz band, consisting of 126 up-link/down-link frequency pair channels. Each frequency pair channel may contain an "X" channel and a "Y" channel, thus increasing channel capacity by using one of two possible pulse-pair spacings on each frequency channel.
The "X" channel pulse pairs are transmitted with a fixed 12.0 .mu.sec spacing between each pulse in the pulse pair. The "Y" channel pulse pairs are transmitted with a fixed 30.0 .mu.sec spacing. This pulse spacing is used to identify the signal as a TACAN/DME signal and to discriminate TACAN systems operating in the "X" or "Y" mode on the same frequency. Any "X" channel signal received exhibiting a pulse spacing outside the range of 12.0.+-.approximately 1.5 .mu.sec is rejected. Any "Y" channel signal received with a pulse spacing outside the range of 30.+-.approximately 1.5 .mu.sec is rejected.
To perform the azimuth determination function, a TACAN ground station continuously transmits thousands of pulse pairs at random intervals each second. These random up-link pulse pairs are called "squitter" signals. The squitter signals are amplitude modulated and are received by a TACAN receiver, commonly referred to as an "interrogator". From the received signals, the TACAN interrogator determines the direction (azimuth) from the station.
The determination of range is accomplished by a process that is different from the azimuth determination function. Each TACAN or DME interrogator transmits sixteen interrogation pulse pairs per second in a random fashion that are received by the TACAN/DME ground station. After a fixed delay, the ground station "replies" with a set of pulse pairs that are received by the interrogator. The aircraft looks for the "replies" to be within a specific "range gate" period of time, and then calculates its distance based on the round-trip travel time of the interrogation/reply sequence.
The 960-1215 MHz TACAN/DME band is used worldwide and is protected for navigation. Conventional TACAN/DME signals are required to meet certain standards and are considered "legal" signals by the International Civil Aviation Organization (ICAO), a United Nations organization that coordinates the use of signals in certain aviation frequency bands.
Different apparatus and numerous methods have been used, or proposed, to provide a data link between a ground station and an aircraft, particularly for the transmission of Differential Global Positioning System (DGPS) signals. The following discusses some of these apparatus and methods.
The most common method is to encode the data on radio frequency (RF) signals using frequency-shift keying (FSK) or binary phase shift keying (BPSK) modulation, or some similar technique, using conventional radios in the VHF and UHF band. While somewhat adequate, the Federal Aviation Administration (FAA) has rejected this approach due to the fact that these bands are not protected internationally for navigational use.
Another possible method is the use of VHF Omnidirectional Ranging (VOR) band signals. Even though protected for navigation, the VOR band is controlled by fixed-based, civil aviation interests and obtaining frequency allocations in this band is extremely difficult.
Another alternate is the use of microwave landing systems (MLS). The MLS band is internationally protected for navigation. The C-band frequency allocation is under demand for other uses and there is currently little commercial use of that portion of the C-band. Consequently, RF components for use in the C-band are scarce and costly making MLS systems relatively expensive. Further, the opportunity for retrofit is almost nonexistent since little MLS equipment exists.
The Mode-S portion of the Air Traffic Control Radar Beacon System (ATCRBS) is also being considered for use as a DGPS data link. Mode-S has an up-link instantaneous data capacity of 4 MHz, and it's spectrum and signals are well understood. However, the DGPS requires a very high degree of continuity of service. Mode-S up-link signals are all transmitted on 1030 MHz. This one frequency is used for all ATCRBS mode A/C/S services by all terminal and enroute Secondary Surveillance Radars. Until recently, these radars were the only users of the up-link frequency, and their narrow beams limited their affect on loading. New users are coming on-line that send up-link signals in an omnidirectional fashion. The Transponder Traffic Alert and Collision Avoidance System (TCAS) uses 1030 MHz, and every TCAS-2 equipped aircraft is capable of sending omnidirectional signals. Mode-S, with omnidirectional up-link transmissions, is being considered for use for Airport Surface Traffic Automation as well. The use of mode-S also results in a high number of critical functions being performed in one box on one frequency. There is risk in relying on the highly crowded asynchronous Mode-S system to relay flight critical DGPS or RGPS data. System compatibility is good with air carriers, since all will eventually have mode-S transponders. However, Mode-S will be slow in reaching prices affordable to general aviation aircraft. As 1030 MHz becomes more crowded, site certification of a Mode-S link for a given DGPS installation will become increasingly more difficult and costly to obtain.
A prototype system designed in the 1980's used modified TACAN signals to up-link DGPS data to an aircraft. This prototype system was named "Sea-Based TACAN and GPS" (STAG). However, the problem with the STAG system was that it added a third pulse to the TACAN pulse pair. The addition of the third pulse increased TACAN system loading and violated the international standards for signals transmitted in the TACAN band. Such a system (indeed, any new system with new signals), would require many years of evaluation and testing to prove interoperability with existing systems, followed by the adoption of a new international standard. Another problem with the STAG prototype system was that none of the airborne or ground STAG equipment could operate concurrently as conventional TACANs. Further, the STAG system made no attempt to reduce errors caused by numerous in-band interferers in whose presence TACAN must operate.
Accordingly, there exists a need for an apparatus and method for encoding and decoding data onto existing navigation signals to provide a data link for the transmission of data from one location to another. A modulation method is needed that encodes data on a TACAN/DME signal without affecting the signal's operation for TACAN/DME range and azimuth determination functions. In particular, there is needed a system for encoding data on conventional existing TACAN/DME signals not violative of ICAO standards. Such a system would eliminate the many years of evaluation and testing required to obtain approval to transmit a new signal in a RF band protected for navigation.
Furthermore, there is needed a system that can be incorporated into existing TACAN/DME ground and airborne equipment without affecting its operation for conventional TACAN/DME functions. Additionally, there exists a need for a method of encoding data on a TACAN/DME signal that reduces the effects of signal interferers that interfere with TACAN/DME signals, thus reducing the bit error rate (BER) of the data link. As such, a system is needed for use as a data link with DGPS and RGPS to provide aircraft with accurate positioning information for approach and landing.